Distribution and Availability of Nitrogen Forms in Soils Containing Ferromanganese Concretion Nodules under Contrasting Land Use Systems in Southern Guinea Savannah

Abstract

This study investigated the distribution and availability of nitrogen forms (total N, organic N, nitrate, and ammonium) in concretion-rich Ferric Luvisols under contrasting land use systems (intensive cultivation vs. forest) across three soil depths (0–15, 15–30, and 30–45 cm). A factorial experimental design (2×3×3) with three replications examined soil only (SOL), concretion only (CON), and soil plus concretion (SCN) samples. Results revealed that land use exerted a stronger influence on nitrogen dynamics than concretion presence. Forest soils maintained significantly higher organic nitrogen (0.95 vs. 0.74 g kg⁻¹), total nitrogen, and
nitrate (0.92 vs. 0.39 g kg⁻¹) levels compared to those under intensive cultivation, reflecting nitrogen capital depletion due to continuous cropping. Intensive land use exhibited 53.5% higher bulk density (1.09 g cm⁻³), lower pH (4.87), and reduced organic matter, creating unfavourable conditions for nitrogen cycling. Findings revealed that concretion nodules contained significantly lower organic and total nitrogen than the soil matrix but exhibited 3 4-fold higher ammonium concentrations, suggesting fixation within oxide structures. Soil plus concretion mixtures showed 30–45% enhancement in total nitrogen compared to soil alone, indicating beneficial interactive effects through organo-mineral associations. Nitrogen distribution remained relatively uniform across depths (0–45 cm), suggesting active vertical movement and leaching vulnerability. Both land use types exhibited low nitrogen fertility status (total N < 1.0 g kg⁻¹), with organic nitrogen dominating (>85% of total N). The study demonstrates that sustainable nitrogen management in concretion-rich Savannah soils require integrated approaches emphasising organic matter conservation, pH amelioration, and practices that address physical, chemical, and biological constraints simultaneously. These
findings provide essential baseline information for developing site-specific fertility management strategies in tropical Savannah agro-ecosystems.